actkbd

actkbd is a keyboard shortcut daemon that works at the system level. It does so through reading the events directly from the input devices, thus working whether a graphical session is running or not.

Do note that this causes issues if it is expected to be used for user-level shortcuts. actkbd is better used to provide global user-agnostic shortcuts like volume or brightness control.

With NixOS, actkbd can be configured using the services.actkbd configuration options.

Finding key codes

The key codes used by actkbd when binding commands will not match with the key code events xev reports.

Obtaining key codes with xev

While logged into an X session, using nix-shell -p xorg.xev --run "xev -event keyboard" will run xev, with filtering for keyboard events.

In the following example output, the line with the key code event is prefixed with an arrow. The keycode for that particular key is 41 with xev.

 |KeyRelease event, serial 28, synthetic NO, window 0x2a00001,
 |    root 0x526, subw 0x0, time 263244557, (457,417), root:(1561,950),
→|    state 0x10, keycode 41 (keysym 0x66, f), same_screen YES,
 |    XLookupString gives 1 bytes: (66) "f"
 |    XFilterEvent returns: False

Tip

While the previous statement may be true, it has been observed that for some, if not all, key code events, subtracting 8 to the xev key code will give the expected actkbd event.

Obtaining key codes with actkbd

To read key codes with actkbd, you will need to select the right input event provider from /dev/input/. Each input device generating input events has its own entry there, enumerated with a number.

To find out which one generates the events from the hotkeys, it is possible, while tedious, to run the following command with each available entry, and see which one generates event notifications when pressing the wanted key.

Alternatively, the lsinput from the input-utils package^{[not upstreamed yet]} allows listing all /dev/input/ files with a useful name.

Another method, which does not require additional packages, will list all the input devices recognized by the X server:

journalctl --unit display-manager.service -b0 | grep "Adding input device" | sed -e 's;.*config/udev: ;;' | sort | uniq

Finally, some device files will also be listed in the /dev/input/by-path/ and /dev/input/by-id/ directories with somewhat recognizable names.

Once the input device found, run the following command (replacing # with the ID) to print the keycodes as actkbd sees them.

nix-shell -p actkbd --run "sudo actkbd -n -s -d /dev/input/event#"

It will report the keys this way:

fKeys: 33
Keys: 14
Keys: 29+46

Where key codes with a + represents combinations. The numbers, including combinations can be used in the configuration.

Sample configuration

The following configuration, from the Backlight page will configure the brightness control keys to use light to control the brightness.

  programs.light.enable = true; # Needed for the /run/wrappers/bin/light SUID wrapper.
  services.actkbd = {
    enable = true;
    bindings = [
      { keys = [ 224 ]; events = [ "key" ]; command = "/run/wrappers/bin/light -A 10"; }
      { keys = [ 225 ]; events = [ "key" ]; command = "/run/wrappers/bin/light -U 10"; }
    ];
  };

Additionally, the sound.mediaKeys.enable option will use actkbd to control the media volumes. See the configuration it generates for an additional example.